The present invention relates to a solid golf ball which has an excellent feel when hit with a golf club and provides excellent control on approach shots.
Of the large variety of golf balls being manufactured today, solid golf balls predominate as a class because they are able to achieve greater distances than thread-wound golf balls. Included among solid golf balls are various types of golf balls, designed for the average amateur golfer having a low head speed, which contain a softer solid core that holds down spin on the ball and increases the distance of travel. Providing a relatively soft solid core is a high priority with designers of golf balls because it improves the feel of the ball, making it possible for golfers, and especially amateur golfers, to experience a soft feel when striking the ball.
Softening the solid core is known to accordingly lower the resilience of the ball. This loss in resilience has hitherto been compensated for by fabricating solid golf balls using a hard cover stock to form the cover enclosing the solid core.
However, little consideration has been given to the spin characteristics of such solid golf balls on approach shots. On an approach shot aimed at the putting green, even striking the ball with an iron fails to impart sufficient spin, making it impossible to achieve the desired distance or adequate control. The result is poor control on approach shots.
It is therefore an object of the invention to provide a solid golf ball which has a very good feel and fully adequate spin characteristics, and which provides excellent control on approach shots.
It has been found that a solid golf ball which has been designed to optimize certain parameters, namely, the deflection of the ball""s core under a static load of 100 kg, the surface hardness of the core, the difference in hardness between the surface and center of the core, and the ratio A/B between the effective surface area of contact A and the apparent surface area of contact B by the golf ball with the club face when the ball is hit with a driver at a head speed of 50 m/s, provides a soft feel and fully adequate spin characteristics, enabling the golfer to achieve excellent control on approach shots.
More specifically, a golf ball provided with a softer core to give the ball as a whole a good feel when hit must have a hard cover enclosing the core in order to assure a good flight performance. While remaining mindful of this need, the inventor has closely examined the overall hardness of the solid core, the difference in hardness between the surface and center of the core, and the surface area of contact by the ball with the club face on impact, referred to below as the xe2x80x9ceffective contact area.xe2x80x9d It has been discovered that a solid golf ball which is designed so that the core has a deflection of 3.5 to 5.5 mm under a static load of 100 kg, the JIS-C hardness at the surface of the core is at least 70, the difference in hardness between the surface and center of the core is at least 10 JIS-C units, and the ratio A/B between the ball""s effective contact area A and its apparent contact area B is from 0.40 to 0.60 provides a soft feel when hit and also exhibits fully adequate spin characteristics on approach shots, ensuring an excellent controllability.
Accordingly, the present invention provides a solid golf ball comprising a core and a cover that encloses the core and bearing a number of dimples in its surface. The core has a deflection of 3.5 to 5.5 mm when subjected to a static load of 100 kg. The core has a JIS-C hardness at the surface of at least 70 and a JIS-C hardness at the center which is at least 10 units lower than the hardness at the core surface. The ball, when hit with a driver at a head speed of 50 m/s, has an effective contact area A and an apparent contact area B with the club face such that the ratio A/B is from 0.40 to 0.60.
xe2x80x9cEffective contact area,xe2x80x9d as used herein and shown in FIG. 1, refers to the surface area of contact which accurately represents those places on the surface of the ball that actually come into contact with the club face. It excludes the scattered places in the same general region of the ball""s surface which do not actually come into contact with the club face, such as dimple recesses. xe2x80x9cApparent contact area,xe2x80x9d as shown in FIG. 3, is used herein to refer to the entire surface area of the general region of the ball that comes into contact with the club face. This quantity includes scattered places within this region where the surface of the ball does not actually come into contact with the club face, such as dimple recesses. That is, when a circular or elliptical region of the ball contacts the club face, the surface area of this circular or elliptical region is the apparent contact area. Subtracting from this apparent contact area the surface area of those places such as dimple recesses where the surface of the ball does not actually come into contact with the club face yields the effective contact area.
Mention is made of xe2x80x9ccontact areaxe2x80x9d in JP-A 7-112036, for example. However, the xe2x80x9ccontact areaxe2x80x9d in these patents denotes the overall surface area computed by such means as elliptical approximation or blacking in of the general region of contact on the ball. This has the same meaning as apparent contact area B used in the present invention, but differs in meaning from effective contact area A as used herein.
In the golf ball of the invention, the use of a cover having a Shore D hardness of not more than 65 to enclose the core is preferable for enhancing the spin characteristics of the ball. Also, the percent of the ball""s surface covered by the dimples formed thereon, referred to hereinafter as the xe2x80x9cdimple surface coverage,xe2x80x9d is preferably at least 71%.
The spin characteristics of the ball when hit with a golf club are closely associated with the surface area of contact by the ball with the club face. In fact, it is described in JP-A 9-135923 that the spin characteristics improve as the contact surface area becomes larger. Hence, golf balls are being designed with a reduced dimple surface coverage to achieve good spin characteristics. Unfortunately, balls designed with a reduced dimple surface coverage cease to make effective use of the dimple aerodynamics, resulting in a decline in the flight performance of the ball. However, it has been found that by setting within a range of 0.40 to 0.60 the above-described ratio A/B between the effective contact area A and the apparent contact area B by the ball on the club face when the ball is hit, the dimple surface coverage can be increased, enabling the dimple aerodynamics to be fully exploited and thus enhancing the flight performance of the ball.
It is recommended that the dimples formed on the surface of the solid golf ball of the invention be of at least two types and have a mean diameter Dm and a mean depth Dp such that the ratio Dm/Dp is from 23 to 30. This enables even more effective use to be made of the dimple aerodynamics, further enhancing the flight performance of the ball.